Preface

This book is about human reproduction. It is a story of sex (of course) and death—death on a massive scale. It is the story of survival through the fierce trials of fertilization and embryonic growth, and the inexorable drive in each of us to perpetuate this cycle by casting our particular twist of genetic fate into future generations. What does it mean, to say that this story is told from an “epidemiologic perspective”? Human biology can be approached in many ways. One is to focus on things that operate consistently in every person. This is the approach of laboratory scientists. However, biology is characterized not just by consistency but by variability. Information on variability requires the study of many persons—the approach taken by epidemiologists. The challenging part of epidemiology is to understand factors that produce human variability—including the reasons some people get a particular disease and others do not. Our detective work can go wrong, as the study of fertility and pregnancy so richly illustrates. There are examples in these pages of misunderstandings due to statistical bias, confounding, and reverse causation. Our errors can be as instructive as the things we get right. Recognizing our mistakes helps us see problems more clearly in our next encounter. Our mistakes also keep us humble. (Is there an epidemiologist alive who has not been snagged by these traps along the way?)